This invention relates to analytical instruments with improved capacities for detecting and determining components in a complex mixture of substances, and more particularly to multichannel analytical instruments such as time-of-flight mass spectrometers with data acquisition means capable of processing a large number of signals within a limited time period. As generally illustrated in FIGS. 1 and 2, mass spectrometers are multistage evacuated devices in which vapors are bombarded with electrons to form positive or negative ions, the mass/charge ratios (m/e) of which are subsequently determined by various means. The data output is often in the form of a mass spectrum which provides a plot of intensity against mass/charge ratio as illustrated in FIG. 3. For a given substance the mass spectrum will consist of mass/charge peaks in number and relative position determined by the fragmentation pathways open to the substance molecules upon ionization. Each substance is associated with a unique set of fragmentation pathways and rate constants, and thus a unique mass spectrum will result and may be used to identify the substance.
In the use of time-of-flight mass spectrometers for sample analysis, a triggered time scan of the type illustrated in FIG. 3 is used to detect electrical signals generated within the instrument which are representative of fragment ion intensities in a mass spectrum with the time axis being converted by computation to the m/e axis of the mass spectrum. Data generated from the time scan are processed by data acquisition or signal processing means to provide a record of the relative intensities and position of the signals in the mass spectrum. The signals corresponding to m/e values are usually irregularly but predictably spaced along the time axis and, particularly for low m/e values, are about 20 nsec FWHM (full width at half maximum). For data acquisition purposes the cycle time period may be divided into a number of narrow time intervals identified as "channels", each corresponding to a position along the m/e axis of the mass spectrum. The number of channels used will vary depending on the time period for each channel and the m/e range to be covered in the mass spectrum. With channels 10 nsec wide, the number of channels needed to collect a mass spectrum over the range 1-1000 atomic mass units (amu) would be 20,000 since a 200 .mu.sec time period would necessarily be recorded. Because the mass signals can be as short as 20 nsec FWHM, it usually is not feasible to use channels wider than 10 nsec.
Many conventional time-of-flight mass spectrometer data acquisition means have no more than four data channels. As a consequence, only one to four data points are recorded each time the complete 200 .mu.sec signal of FIG. 3 is generated at the ion multiplier output which represents one cycle. After each cycle of the TOFMS, the data acquisition channels are moved to a new location along the time axis, relative to some trigger pulse that initiates a cycle, so that bit by bit the entire 200 .mu.sec time period is recorded. A true representation of the output is not recorded if the ion concentrations change, as they usually will, during the time this instrument is repeatedly cycled to construct a complete spectrum. In addition to the lengthy data processing time, the large number of TOFMS cycles requires an appreciable amount of instrument time.
More recently, data acquisition means have been provided with 2048 channels to record data from a time-of-flight-mass spectrometer. When the channels are spaced in time at 10 nsec intervals, the instrument is still only capable of recording a 20 .mu.sec (2048.times.10 nsec) portion of the output illustrated in FIG. 3 during one cycle. Therefore, it is necessary to repeat the process 10 times with a variable time delay triggering the data acquisition means, in order to completely record the 200 .mu.sec long mass spectrum.
In addition to the limitations of the scanning process, othe problems with the time-of-flight-mass spectrometers have been related to the inability of the data acquisition portion of the instrument to record an individual signal of short duration at each channel and the lack of capacity to process a multiplicity of signals within a limited time. These problems have also been related to the capacity of the instrument to perform the desired function with a reasonable limit on the complexity and cost of the electronics.
One object of this invention is an analytical instrument useful for generating and detecting a large number of signals of relatively short duration and data acquisition means capable of processing those signals. A second object is an analytical instrument with data acquisition means capable of processing a large number of signals within a limited time. A third object of the invention is an analytical instrument capable of processing a plurality of predetermined signals. A fourth object of the invention is an analytical instrument characterized by a large number of analog signals generated in a single time scan within a short period and capable of processing all of the data represented by the signals within essentially one time scan. Another object of the invention is a data acquisition means in an analytical instrument capable of processing a large number of signals with limited equipment. Yet another object of the invention is data acquisition means in an analytical instrument capable of processing analog signals of a duration time less than about 50 nsec. An additional object of the invention is a time-of-flight mass spectrometer capable of generating a mass spectrum over the range 1-1000 amu or selected portions thereof. A further object of the invention is an instrument capable of summing the spectra acquired over many scans.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.